Electrolytic capacitor



NOV. 4, R. D. BUGEL ELECTROLYTIC CAPACITOR Filed March 1, 1951 ALUMINUMPLUS IMPURITIES CU, FE, SI

ELECTROLYTE DIELECTRIC AL O INVENTOR ROELOF DIRK BUGEL AGENT PatentedNov. 4, 1952 1 UNITED STATES PATENT OFFICE ELECTROLYTIC CAPACITORApplication March 1, 1951, Serial No. 213,367 In the Netherlands March22, 1950 3 Claims. 1

This invention relates to electrolytic capacitors in which at least oneelectrode consists of aluminium coated with an oxide layer which fulfilsthe function of a dielectric. If the capacitor is intended for use withalternating current, it must comprise at least two such electrodes. Thecounter-electrode used is an electrolyte, for example on the basis ofboric acid and glycol, which may be provided in the liquid state or inthe form of a paste.

It is known to utilise aluminium of great purity for the saidelectrodes, for example aluminium containing less than some hundredthsper cent of impurities. With the use of such material capacitors may bemade having a low leakage current which increases slightly duringoperation, such capacitors consequently having a sufficient life.Impurities in the aluminium have been found detrimentally to affect thedielectric properties of the aluminium-oxide layer due to the formationof oxides of other metals in this layer. This view has led to the use ofaluminium of high purity for the manufacture of electrolytic capacitors.

It is common practice to subject the electrode material to an etchingoperation in order to ensure a high capacitance per unit-volume. It hasbeen found, however, that according as the aluminium has a greaterpurity, the etching operation involves greater difficulties, it being inpractice impossible for aluminium of 99.99% purity to be etched to asufficient extent.

It follows therefore that concerning the life of the capacitor and theetching treatment of the electrode material, the requirements for thepurity of the aluminium conflict with one another. In addition, theconsumption of energy in forming the oxide layer decreases with higherpurity of the kinds of aluminium commercially sold.

The figure of the drawing is a cross section of a part of anelectrolytic capacitor.

During the investigation which has led to the present invention it hasbeen found that the in fluence exerted by various impurities occurringin commercial aluminium upon the etching and forming treatments is notthe same. For the sake of completeness, it is mentioned that someelements such as magnesium, manganese and titanium in the form ofimpurities in or small additions to the aluminium have no detrimentaleffect on capacitor life, as is, however, the case with the impuritiesnormally occurring in practice, which are substantially iron and siliconand, to a smaller extent, copper. Probably as a result 2 of thepurificafion methods utilised, the content of iron is higher than thecontent of silicon even in the very pure kinds of aluminium which arecommercially sold.

It has been found that the iron contained in aluminium is advantageousfor the etching treatment, but detrimental to the process of forming theoxide layer, whereas silicon is harmless to the forming process and,although to a smaller extent than iron, facilitates the etchingtreatment. When disregarding copper as an impurity, since the content ofcopper is only some thousandths per cent even in commercial kinds ofaluminium, which contain in total about 0.2% impurities, it appears thatin regard to the composition a compromise is possible in which theabove-mentioned disadvantages occurring in commercial kinds of aluminiummay be decreased by a suitable choice .of the ratio between iron andsilicon.

According to the invention, an electrolytic capacitor comprising atleast one aluminium electrode provided with an oxide layer, ischaracterized in that this electrode contains less than 0.01% copper,the content of iron being comprised between 0.03% and 0.08% and thecontent of silicon being from 0.01% to 0.05% higher, preferably at least0.02% higher, than the iron content.

Highly satisfactory results are obtained with aluminium having an ironcontent between 0.03 and 0.06% and a silicon content between 0.05% and0.09%, and being the greater the higher the iron content.

In view of the fact that the conventional methods of purifying aluminiumlead to products, the iron content of which is higher than the siliconcontent, it will. as a rule, be necessary to add a small amount ofsilicon to the commercial purified aluminium.

Electrolytic capacitors comprising electrodes of aluminium containingiron and silicon in the quantities indicated above, are substantiallyidentical as regards life and quality with capacitors in which aluminiumof 99.99% purity is utilised as the electrode material, whilst in regardto the etching treatment during manufacture a considerable advantage isobtained with respect to the last-mentioned capacitors. This will bemade clear with reference to some examples in the table followinghereinafter.

In this table, the etching period, the forming period, the sum of theseperiods and the forming energy are indicated for some kinds of aluminiumfoil of a thickness of 250 microns in which the ratios between the ironand silicon contents are different and which, as is usually the case,have been heated at 300 C. for about 10 hours. The etching period chosenis the time in minutes required to ensure etching up to 50% of theinitial weight at a temperature of 0 C. in a 10% solution ofhydrochloric acid in which 30 gmsfiof aluminum'aredissolvedzper litre.The forming period indicated is the time in minutes required for formingan oxide layer by anodic oxidation with the use of a conventionalsolution of boric acid and borate (for example a solution containing 4%of boric acid and-0.14% of borax) until the current de'n'sityfiat avoltage.

of 400 volts has decreased to aboutramamps/dmfii- The forming energyindicated in the table'isto -be understood to mean the energykilowatthours consumed during the forming period 'for 25 foils having asurface area of 50 cm. each.

The table shows that the total time required fforthe etching and formingtreatmentsiis lowest 'in 'Example 2. Furthermore, it appears that thealuminium of greater purity in Example 3v as compared with that inExample '2 .ryields .only little decrease in consumption of energy,whilst. as comparedwithiExample.1-which does not satisfy the conditionsof "the invention"in' this. case a decrease lin consumption of energy.is achieved,

tween 003% and-0.08% iron, and an amount of silicon "which 'is 0.02%greater than the amount of iron.

=3.Anaelectrode material for an electrolytic capacitorwhich consists ofaluminum of greater than 99% purity, less than .01% copper, between0.03% and 0.06% iron, and between 0.05% and 0.09% silicon.

RO-EBOF DIRKBUGEL.

REFERENCES CITED 'ihe'fdllowing'meferences are of record in the "file ofthis ,patentz UNITED STATES PATENTS Number Name -'Date 1578,85?iSiegmund Mar. 30, "1926 1,710,073 Ruben Apr. 1-3, 1'929 2,024,210Edelman Dec. 17, 1935 OTHER REFERENCES "Electrolytic Gondensers 'by'PhilipiR-zfloursey,

published by Chapman -'& HallLtd., London, .1937,

page-.52.

1. AN ELECTRODE MATERIAL FOR AN ELECTROLYTIC CAPACITOR WHICH CONSISTISOF ALUMINUM OF GRETER THAN 99% PURITY, LESS THAN 0.01% COOPER, BETWEEN0.03% AND 0.08% IRON, AND AN AMOUNT OF